An example of a liquid level measurement device is disclosed in the document WO2005038415. The device comprises a transducer arranged outside the liquid for transmitting and receiving acoustic signals, and a waveguide connected to the transducer and extending into the liquid. In the basic operation of the device, an acoustic signal is transmitted from the transducer into the waveguide, which signal is reflected by the surface of the liquid into which the waveguide extends. By knowing the transit time and the velocity of the signal, it is possible to calculate the liquid level.
It in a known fact that the velocity of acoustic signals is dependent on for example temperature, which may render the measurement erroneous. To this end, the device in WO2005038415 comprises a reference system, wherein the acoustic signal travels a known distance in order to determine the current speed of sound, whereby the current speed of sound is used to calculate the liquid level.
However, for example when the liquid level is low, there may be a significant temperature difference throughout the measurement device, making the measurement erroneous. This even though a reference system is used, since the temperature in the reference system may differ from that of other parts of the measurement device.